The present invention relates to a tapered roller bearing which can be used with e.g. a driving device for a railroad wheel.
For supporting gears of a driving device for a railroad car, generally, tapered roller bearings are used which can bear relatively large radial and axial loads.
As shown in FIGS. 8 and 9, in a driving device 1 for a railroad car, power is transmitted from an output shaft 3 of a main electric motor 2 to an axle 7 supporting wheels 6 through a pinion 4 and a gear wheel 5. A gear shaft 8 carrying the pinion 4 is rotatably supported by a pair of tapered roller bearings 9. Compared with the gear wheel 5, the pinion 4 undergoes a high number of revolutions and a large load. The tapered roller bearings 9 are used in a harsh environment. Even in such an environment, superior rolling fatigue life and reliability are required.
Hereinbelow, description is made of a conventional tapered roller bearing. FIG. 14 is an enlarged sectional view of a conventional tapered roller bearing 100 which comprises an outer ring 110, an inner ring 120, tapered rollers 130 and a retainer 140. The outer ring 110 has a conical raceway 111 on the inner peripheral surface. The inner ring 120 has a conical raceway 121 on the outer peripheral surface, and a large flange surface 122 on the large-diameter side of the raceway 121 and a small flange surface 123 on the small-diameter side. The tapered rollers 130 are mounted between the raceway 111 of the outer ring 110 and the raceway 121 of the inner ring 120, and each has a conical rolling surface 131 on its outer peripheral surface and a large end face 132 and a small end face 133. The retainer 140 rollably houses a large number of the tapered rollers 130 in pockets 141 formed at regular spacings.
As shown in FIG. 15, the apexes of the raceway 111 of the outer ring 110, of the raceway 121 of the inner ring 120, and of the rolling surfaces 131 of the tapered rollers 130 converge to one point O on the central axis x of the tapered roller bearing 100. While the bearing is rotating, the tapered rollers 130 are pressed against the large flange surface 122 of the inner ring 120 by the combined force applied from the raceway 111 and the raceway 121 and undergo a rolling motion on the raceways with their large end faces 132 guided by the large flange surface 122. During rotation of the bearing, the small end faces 133 of the tapered rollers 130 do not contact the small flange surface 123 of the inner ring 120, such that there is a slight gap present therebetween.
With tapered roller bearings used in a driving device for a railroad car, especially in winter or in a cold district, low-temperature starting performance is important. That is to say, when a car, which has been stopped during a nighttime, begins to travel early in the morning, since the driving device and the bearings in the driving device are accelerated (operate) from a sufficiently cooled state, in the bearing 100, only the temperature of the inner ring 120 rapidly rises with the temperature of the outer ring 110 not following. Thus, the difference in temperature between the inner and outer rings of the bearing can be large and the initial bearing gaps disappear, so that if oil film formation at the contact portions between the large flange surface 122 of the inner ring 120 and the tapered rollers 130 is bad, the lubricating condition worsens and the bearing 100 can seize.
In particular, with a tapered roller bearing used in a driving device for a railroad car, the lubricating arrangement is such that gear oil lubricating the pinion 4 and the gear wheel 5 is splashed on the bearing making use of the rotation of the pinion 4 and the gear wheel 5. Thus, in winter or in a cold district, since the viscosity of the gear oil increases due to coldness during a nighttime, oil formation at the contact portions between the large flange surface 122 of the inner ring 120 and the large end faces 132 of the rollers 130 worsens, so that the lubricating condition tends to worsen and the bearing 100 seizes.
Also, if the contact state between the raceways 111 and 121 of the outer ring 110 and the inner ring 120 and the rolling surfaces 131 of the tapered rollers 130 is bad, skew of the tapered rollers 130 tends to grow, and the contact surface pressure at the inner ring large flange surface 122 becomes excessive, so that seizure occurs.
In order to solve these problems, in view of the fact that the oil film forming properties at the contact portions between the inner ring large flange surface and the rollers of the tapered roller bearing depend on their surface roughness, a tapered roller bearing is proposed in which the average roughness (R)[=(Rr2+Rb2)xc2xd] of the roughness (Rr) of the roller large end faces and the roughness (Rb) of the large flange surface of the inner ring is set at 0.14 xcexcm Ra or less (JP patent publication 11-210765).
Also, in view of the fact that the surface pressure at the contact portions between the large flange surface of the inner ring and the tapered rollers depends on the positions of the contact portions therebetween, a tapered roller bearing is also proposed in which the radius of curvature R of the roller large end faces is set at 75-85% of the roller reference radius R0, and the central angle of the contact positions between the tapered rollers and the inner ring large flange surface will be {fraction (1/15)} to {fraction (1/10)} of the roller angle (JP utility model publication 5-87330).
But even if both of these solutions are used, it is impossible to completely prevent seizure of the bearing. Thus, confusion occurred in controlling the diagram of the railroad cars. Therefore, tapered roller bearings that are free of seizure in the bearings are strongly desired.
Thus, a first object of the present invention is to provide a tapered roller bearing which does not seize even in a severe winter or in an extremely cold region.
Another conventional tapered roller bearing used in power transmitting devices for vehicles such as ring gear support portions of differentials or shaft support portions of transmissions is shown in FIG. 16A. It includes an outer ring 222 having a conical raceway 221, an inner ring 226 having a conical raceway 223 and provided with a large flange surface 224 on the large-diameter side of the raceway 223 and a small flange surface 225 on the small-diameter side, a plurality of tapered rollers 227 rollably arranged between the raceways 221 and 223 of the outer ring 222 and inner ring 226, and a retainer 228 for retaining the tapered rollers 227 at predetermined equal circumferential distances. The distance between the large flange surface 224 and the small flange surface 225 on the inner ring 226 is designed slightly longer than the length of the tapered rollers 227.
The tapered rollers 227 are designed such that they are in line contact with the raceways 221 and 223 of the outer ring 222 and the inner ring 226, and the cone angle apexes of the tapered rollers 227 and the raceways 221 and 223 coincide on a point O on the central axis of the tapered roller bearing. With this arrangement, the tapered rollers 227 can undergo a rolling motion along the respective raceways 221 and 223.
With this tapered roller bearing, since the cone angles of the raceways 221 and 223 are different, the combined force of the loads applied from the respective raceways 221, 223 to the tapered rollers 227 acts in such a direction as to push the tapered rollers 227 toward the large flange surface 224 of the inner ring 226. Thus, during use of the bearing, the tapered rollers 227 are guided with their large end faces 229 pressed against the large flange surface 224, so that the large end faces 229 and the large flange surface 224 make sliding contact with each other.
On the other hand, since the distance between the large flange surface 224 and the small flange surface 225 of the inner ring 226 is designed slightly longer than the length of the tapered rollers 227, as shown enlarged in FIG. 16B, the small flange surface 225 does not contact the small end faces 230 of the tapered rollers 227 such that small gaps are present therebetween. Also, the small flange surface 225 is formed as a surface inclined outwardly at an angle relative to the small end faces 230.
With this tapered roller bearing, it was usual that the radius of curvature R of the large end faces 229 of the tapered rollers 227 were set at 90-97% of the reference distance Rb (that is, the distance from the apex of the cone angle of the tapered rollers 227 to the large flange surface 224 of the inner ring 226). Also, heretofore, the surface roughness of the large flange surface of the inner ring was set at 0.1-0.3 xcexcm.
With such a tapered roller bearing, axial pre-load is loaded to prevent axial movement of the tapered rollers 227 during use and stably cause the tapered rollers 227 to make line contact with the raceways 221 and 223 of the outer ring 222 and the inner ring 226.
But with use, the flange is gnawed by metallic contact between the large end faces 229 of the tapered rollers 227 and the large flange surface 224, and contact between the edge portions of the large end faces 229 and the large flange surface 224 during skewing of the tapered rollers 227. This results in release of pre-load, a phenomenon in which the pre-load decreases gradually.
As for the relation between the preload and the bearing life, as shown in FIG. 17, while there exists a pre-load of a suitable magnitude (region in which the axial clearance is negative), the life ratio L/L0 (wherein L is a life considering the clearance and pre-load, and L0 is the life when the clearance is 0) is 1 or over. But when the axial clearance changes from zero to the positive region, release of pre-load occurs, so that the life ratio gradually decreases.
A second object of this invention is to provide a tapered roller bearing which has a longer life by preventing release of pre-load.
According to the present invention, there is provided a tapered roller bearing comprising an inner ring having a raceway on its outer peripheral surface, an outer ring having a raceway on its inner peripheral surface, a large flange surface and a small flange surface, tapered rollers rollably mounted between the raceway of the inner ring and the raceway of the outer ring and having a large end face at one end thereof and a small end face at the other end thereof, and a retainer for retaining the tapered rollers at regular spacings, wherein the average roughness R[=(Rr2+Rb2)xc2xd] of the roughness Rr of the large end faces of the tapered rollers and the roughness Rb of the large flange surface of the inner ring is 0.14 xcexcm Ra or less, wherein the radius of curvature R of the large end faces of the tapered rollers is 75 to 85% of the roller reference radius, and the central angle of the contact positions between the tapered rollers and the large flange surface of the inner ring is set at {fraction (1/15)} to {fraction (1/10)} of the roller angle, wherein the center of the contact surface between the tapered rollers and the raceways is offset toward the large end faces of the tapered rollers from the center of the rolling surfaces of the tapered rollers by an amount which is 10% or less of the effective length of the tapered rollers.
With the tapered roller bearing described above, because the average roughness R[=(Rr2+Rb2)xc2xd] of the roughness Rr of the large end faces of the rollers and the roughness Rb of the large flange surface of the inner ring is set at 0.14 xcexcm Ra or less, oil formation at the contact portions between the roller large end faces and the inner ring large flange surface is ensured. Also, because the radius of curvature R of the roller large end faces is 75 to 85% of the roller reference radius R0, and the central angle of the contact positions between the tapered rollers and the large flange of the inner ring is set at {fraction (1/15)} to {fraction (1/10)} of the roller angle, the sliding speed of the rollers relative to the sliding portions between the roller large end faces and the inner ring large flange surface decreases, so that the turning torque and the bearing heat buildup decrease. Further, because the center of the contact surface between the tapered rollers and the raceways is offset toward the roller large end faces from the center of the rolling surfaces of the tapered rollers by an amount which is 10% of their effective length, skew of the tapered rollers is suppressed. Also, due to their combined effects, seizure of the bearing is prevented.
According to the present invention, the rolling surfaces of the tapered rollers are crowned and the shape of the crowning at the large end face side is different from that at the small end face side of the tapered rollers.
By providing crowning on the rolling surfaces of the tapered rollers, it is possible to offset the center of the contact surface between the tapered rollers and the raceways toward the roller large end faces from the center of the rolling surfaces of the tapered rollers within a range of 10% of their effective length.
According to the present invention, the raceways are crowned so that the shape of crowning at the large flange side of the inner ring is different from the shape of crowning at the small flange side of the inner ring.
With this arrangement, it is possible to offset the center of the contact surface between the tapered rollers and the raceways toward the roller large end faces from the center of the rolling surfaces of the tapered rollers within a range of 10% of their effective length.
According to the present invention, with the contact surface between the tapered rollers and the raceways at the center of the raceways of the tapered rollers, the angle of the raceway of the outer ring is set smaller and the angle of the raceway of the inner ring raceway is set larger.
With this arrangement, too, it is possible to offset the center of the contact surface between the tapered rollers and the raceways toward the roller large end faces from the center of the rolling surfaces of the tapered rollers within a range of 10% of their effective length.
By any of the above arrangements, it is possible to offset the center of the contact surface between the tapered rollers and the raceways toward the roller large end faces from the center of the rolling surfaces of the tapered rollers within a range of 10% of their effective length, and thus it is possible to provide a tapered roller bearing in which skew of the tapered rollers is suppressed and seizure of the bearing will not occur even in a very cold season or in a cold region.
According to this invention, there is also provided a tapered roller bearing which comprises an outer ring having a conical raceway, an inner ring having a conical raceway and formed with a large flange surface on the large-diameter side of the conical raceway and a small flange surface on its small-diameter side, a plurality of tapered rollers rollably arranged between the conical raceway of the outer ring and the conical raceway of the inner ring and having a large end face and a small end face, and a retainer for retaining the tapered rollers at circumferential spacings, wherein during use of the bearing, the large end faces of the tapered rollers are guided in contact with the large flange surface of the inner ring, wherein a predetermined pre-load is imparted between the tapered rollers and the respective raceways of the outer ring and the inner ring, wherein the radius of curvature R of the large end faces of the tapered rollers is set at 75 to 85% of the reference distance Rb between the apex of cone angle of the tapered rollers and the large flange surface.
Other features and objects of the present invention will become apparent from the following description made with reference to the accompanying drawings, in which: